1. Field of the Invention
The present invention relates to a developing device mounted to an image forming apparatus such as a copier, a facsimile machine, or a printer and an image forming apparatus including the same, and more particularly, to a developing device which uses a two-component developer made of a magnetic carrier and toner.
2. Description of Related Art
Up to now, as development methods using dry toner which are employed in an image forming apparatus using an electrophotographic process, there are known: a one-component development method which does not use a carrier; and a two-component development method in which a two-component developer for charging non-magnetic toner by using a magnetic carrier is used to develop an electrostatic latent image on an image bearing member (photosensitive member) by a magnetic brush made of toner and a carrier and formed on a developing roller.
The one-component development method is suitable for achieving higher image quality because the electrostatic latent image on the image bearing member is not disturbed by the magnetic brush. However, on the other hand, the toner is charged by a charging roller with a layer thickness on the developing roller being regulated by an elastic regulating blade, and hence an additive to the toner adheres to the charging roller and lowers a charging ability thereof, which makes it difficult to maintain a toner charge amount with stability. Further, the toner sometimes adheres to the regulating blade, resulting in non-uniform layer formation, which causes an image defect.
Further, in a case of color printing for performing color superimposition, because transparency is required for color toner, the color toner needs to be non-magnetic toner. Therefore, a full-color image forming apparatus often adopts the two-component development method of charging and transporting toner by using a carrier.
Incidentally, a developing device generally used in the two-component development method is of a two-axis transport type and includes: a developer supplying/collecting portion in which a stirring-transport screw for supplying the developer to the developing roller and transporting the developer collected from the developing roller is located; and a stirring-transport portion for replenishing the developer transported from the developer supplying/collecting portion with the toner and stirring and transporting the replenished developer.
However, in the above-mentioned two-axis transport type, the developer that has been used for development of the electrostatic latent image on the photosensitive member and has a low ratio (T/C) of the toner to the carrier is collected by the developer supplying/collecting portion and immediately carried onto the developing roller again. This raises a problem that a stable image density cannot be obtained.
In particular, in a case where an image having a high density in part such as a patch image is printed, it is likely to cause a phenomenon called development hysteresis in which a part exhibiting a low toner density occurs in the developer and the previous image appears as an after image (ghost).
As a countermeasure against this, for example, there is known a developing device of a three-axis transport type including: a developer supplying-transport portion for supplying the developer to a developer carrying member (developing roller); a developer collecting-transport portion for transporting the developer collected from the developer carrying member; and a developer stirring-transport portion for replenishing the developer transported from the developer collecting-transport portion with the toner and stirring and transporting the replenished developer, which are separately provided.
According to the above-mentioned method, all the developer with the toner density (T/C) lowered after development is collected by the developer collecting-transport portion and is not immediately used for development, which allows a stable image density to be obtained.
However, in the above-mentioned method, the developer with a low toner density, which has been collected from a surface of the developer carrying member and has been transported through within the developer collecting-transport portion, is merged into the developer within the developer supplying-transport portion at an end portion of the developer supplying-transport portion. Hence, in particular, a high-speed machine exhibiting a high speed of transporting a developer is liable to have the insufficient stirring therein, which causes malfunctions that the toner density (T/C) within the developer is partially lowered and that lowered flowability inhibits the developer from being passed to the developer supplying-transport portion.
Further, as nearing a downstream side of the developer carrying member in the above-mentioned structure, a developer amount within the developer supplying-transport portion decreases, and the developer decreases in volume. Hence, an ability to scoop up the developer may become insufficient in a downstream side portion of the developer carrying member, thereby lowering the image density partially.
In addition, in a case where a toner density sensor and a developer discharge portion are located in the vicinity of a downstream side of the confluence at which the developer is merged, there is a fear that there may occur erroneous detection of the toner density or excessive discharging of the developer due to irregularity caused in the volume of the developer.